Add googletest as a submodule in third_party/googletest.

Add implementations of open(2), mkdir(2), stat(2),
etc. that support long paths under Windows (paths
longer than MAX_PATH in <windows.h>, which is 260
characters).

The implementations are in a separate namespace
(google::protobuf::internal::win32), so they won't
collide with the standard implementations in
<io.h>, but after importing them with `using` they
can be drop-in replacements.

Fixes https://github.com/bazelbuild/bazel/issues/2634
Fixes https://github.com/google/protobuf/issues/2891

The check target did not depend on the test_plugin binary, so this would
cause "make check" to fail at times because of the absence of
test_plugin. This change adds a dependency on test_plugin so that it
will always get built before the tests are executed.

A series of improvements:

- Improved Protobuf module compatibility (disabled by default);
- Hide advanced settings;
- Added build tree configuration;
- Added build of examples.

Change-Id: I6fa9f1b65d6c06b891aabf3f3d868364d74d727d

Build tests optimization
Now lite-test added to CTest collection

Change-Id: Ieb7a2c2fbf35bc2a8fa65b915a5ecb68c83863e4

[ci skip]

Change-Id: I9391c09640e0b0b2b21c45a97a1fc91814d95c5d

Escape characters don't count for string literal size, no need to pre-generate escape string.

Added unit test to touch enormous cpp generated descriptor.

Updated makefile to include enormous_descriptor.proto

Fixed language compatibility error.